1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image using an electrophotographic method, and more particularly, to an image forming apparatus, such as a copying machine, a printer, a facsimile machine, and a multifunction peripheral, including an endless belt stretched around stretch rollers.
2. Description of the Related Art
An electrophotographic color image forming apparatus includes pressure contact members such as a primary transfer roller and a secondary transfer roller. Each of the pressure contact members is urged by an urging member to press and contact a surface of an endless belt such as an intermediate transfer belt with a predetermined pressure. If a long time period has elapsed in a state that these pressure contact members are pressing and contacting the intermediate transfer belt, a creep phenomenon occurs. This causes plastic deformation such as a curl of the intermediate transfer belt.
Generally, at a secondary transfer nip, since a relatively high transfer bias is applied to form a transfer electric field on a transfer material such as paper through which it is difficult to conduct electricity. Thus, abnormal discharge caused by the high voltage needs to be prevented at the secondary transfer nip. Accordingly, the secondary transfer roller is arranged to be offset on an upstream side in a sheet conveyance direction with respect to a secondary transfer opposing roller inside the intermediate transfer belt. At that time, there is an area in which only the intermediate transfer belt and the secondary transfer roller contact each other. Such an area is called a tension nip area.
Moreover, a pressing force of the secondary transfer roller is set relatively high to enhance adhesiveness between the secondary transfer roller and the intermediate transfer belt. In a case where the tension nip area is left for a long time in a state of being pressed and contacted with a high pressure, a curl occurs on the intermediate transfer belt with a recess toward the outside. The curl of the intermediate transfer belt reduces not only adhesiveness between a photosensitive drum and the intermediate transfer belt at a primary transfer nip portion, but also adhesiveness between the intermediate transfer belt and the secondary transfer roller at a secondary transfer nip portion. Such reduction in adhesiveness causes generation of an abnormal image such as transfer unevenness.
Accordingly, the secondary transfer roller needs to be separated from the intermediate transfer belt while the intermediate transfer belt is stopped, for example, when the image forming apparatus is in a power-off state or a print standby state, to solve such a problem. Various configurations have been discussed to separate the secondary transfer roller from the intermediate transfer belt.
Japanese Patent Application Laid-Open No. 2001-296716 discusses a configuration in which a stretching member and a secondary transfer member, arranged inside an intermediate transfer belt to apply tension to the intermediate transfer belt, are separated from the intermediate transfer belt.
In Japanese Patent Application Laid-Open No. 2001-296716, however, a reciprocating rod for causing the secondary transfer member to contact and be separated from the intermediate transfer belt and a horizontal rod for causing a primary transfer member to contact and be separated from the intermediate transfer belt are different members. Consequently, such a configuration obstructs further reduction in size and space of a transfer unit and the image forming apparatus.